This invention relates to retro-reflective sheet material (also known as reflex reflectors) and to a method of preparing the same.
Retro-reflective material is well known and widely used in the prior art, mainly in connection with road signs and automobile licence plates and the like. The material is capable of reflecting an incident beam of light back in the general direction of the light source. When, for example, a car headlight illuminates a road sign bearing such material in otherwise dark surroundings, the sign reflects a greater amount of light towards the driver than surrounding objects and is therefore clearly visible.
Known retro-reflective materials fall into two general types. The first type consists of spherical glass beads adhered to the surface of a transparent organic coating applied over a reflective metal (or other) substrate but only partially imbedded in it so that a glass-air interface is presented to incoming light. This type of material does not function as a retro-reflector when the surface is wet with water.
The second type of material consists of high refractive index (about 1.9 and greater) glass beads suspended within a relatively thick film of a transparent organic coating (plastic film) applied over a reflective metal substrate. Such material retains its retro-reflective properties when wet.
Theoretical considerations dictate that the most effective retro-reflectors of the second type can be formed by positioning the glass beads at just the right distance from conforming segments of concave spherical mirrors of corresponding size located behind the individual beads. This optimum distance will vary depending on the diameter of the beads, their refractive index, and the refractive index of the medium in which they are suspended.
Various known retro-reflectors embody such a structure but have generally been produced in the form of flexible tapes or sheets which are adhered to the desired object, such as a road sign. The step of adhering the tape to the object can be time consuming and uneconomical, and the tape may peel from the object after a period of exposure to the elements.
One example of known retro-reflectors is disclosed in U.S. Pat. No. 2,407,680 issued on Sept. 17, 1946 and assigned to Minnesota Mining and Manufacturing Company. This patent was one of the first to disclose the second type of structure referred to above employing high refractive index spheres, and it is to be noted that it suggests the use of a polished metal surface as the back reflector with the beads spaced an optimum distance therefrom. The patent also suggests the formation of concave mirrors in a reflective surface formed by a reflective binder layer for the heads.
U.S. Pat. No. 2,543,800 issued on Mar. 6, 1951 and assigned to Minnesota Mining and Manufacturing Company discloses a retro-reflector in which the beads are spaced a small distance from corresponding reflector surfaces formed by pressing the beads partially into a moldable cushion layer having a reflective surface coating containing metallic flake pigment particles, the beads being spaced from the cushion layer by a thin film which may contain a transparent pigment. After the pressing operation the plastic layers are cured. One disadvantage of such retro-reflectors is that the reflector surfaces formed by the cushion layer are not as reflective as a polished metal surface and therefore light is lost at these surfaces.
U.S. Pat. No. 3,922,433 issued on Nov. 25, 1975 and assigned to Aluminum Company of America relates to partially embedding the spherical glass beads into a metallic coating while it is in the molten condition. This invention is an attempt to form a retro-reflective surface directly on a substrate made of a hard material, such as a road sign, without first forming a flexible tape to be adhered thereto. An iron-base substrate is dipped into a molten bath of aluminum, zinc, tin, lead or alloys thereof and is sprayed with the glass beads by an air gun as the substrate is withdrawn from the bath and the coating is still molten. This method has the disadvantages that it is expensive and the beads are not spaced from the reflective surface as is required for the optimum retro-reflection.
There is therefore a need for a method of producing a retro-reflective surface directly onto hard metal substrates, which method permits the beads to be spaced by the optimum distance from conforming concave reflective surfaces.
The formation of a plastic layer containing the glass beads and the pressing of the beads into the metal substrate has been contemplated but, because of the relative hardness of the substrate surface, it has been found that the beads tend to shatter and the layer of plastic between the beads and the substrate surface tends to become attenuated or damaged when the plastic layer is rolled with sufficient force to cause the beads to indent the substrate surface. Moreover, the beads tend to become mis-aligned with the indentations in the surface so that a useless product is produced.